The British government has ordered more than 100 schools to keep some or all of their buildings closed when the new academic year begins next week because of concern that crumbling concrete threatens the safety of children and staff. The announcement late Thursday sent school administrators scrambling to find ways to accommodate pupils, with some expected to return to the online instruction used during the COVID-19 pandemic. The timing of the decision, just days before the start of classes, raised questions from parents and school officials about why the government didn’t act sooner.